Resistivity and thermo-electric behaviour of some mixed-valence systems at high pressures and high temperatures

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Resistivity and thermo-electric behaviour of some mixed-valence systems at high pressures and high

temperatures

T. Ramesh, V. Shubha

To cite this version:

T. Ramesh, V. Shubha. Resistivity and thermo-electric behaviour of some mixed-valence systems at

high pressures and high temperatures. Journal de Physique Colloques, 1979, 40 (C5), pp.C5-344-C5-

345. �10.1051/jphyscol:19795119�. �jpa-00218903�

JOURNAL DE PHYSIQUE

Colloque C5, supplément au n° 5, Tome 40, Mai 1979, page C5-344

Resistivity and thermo-electric behaviour of some mixed-valence systems at high pressures and high temperatures

T. G. Ramesh and V. Shubha

Materials Science Division, N.A.L., Bangalore-560017, India

Résumé. — Dans cet article nous présentons quelques résultats nouveaux sur les propriétés de transport de SmS et Sm0 8Tb0 2S aux fortes pressions. Utilisant le pouvoir thermo-électrique, nous avons trouvé que la ligne de changement de phase pour la transition métal-semiconducteur se termine à un point critique pour SmS. En outre, le comportement de Sm0 8T b0 2S aux fortes pressions indique que les ions Sm sont dans l'état complètement trivalent en dessus de 12 kbar.

Abstract. — This paper reports some new data on the transport properties of SmS and Sm0 gTb0 2S at high pres- sures. Using thermo-power as a tool, we have established that the semiconductor-metal phase boundary in SmS terminates at a critical point. Further the high pressure behaviour of Sm0 8T b0 2S suggests that at pressures greater than 12 kbar, the Samarium ion is in the fully trivalent state.

1. Introduction. — The electronic transformation | 30°c in rare-earth systems has received considerable atten- +20 - ^^-~- *

tion in recent years [1]. The semiconductor-metal / " ^ 535t;

transition in SmS in the P- T plane has been studied by

i A i ^-r i i . Jayaraman et al. [2] up to 250 °C and by Tonkov 4 | 8 X _ 16 20 2k 28 and Aptekar [3] up to 500 °C. The present study / PRESSURE (kbar) involving thermo-power and resistivity as tools is ° " I ^ ^ 7 1 5 C

aimed at establishing the critical point for S-M f phase boundary. Further the high pressure behaviour <3 " ^ - I /

of thermo-power and resistivity in chemically col- ^ I / lapsed Sm

Tb

S suggests that at high pressures the -± .grj. I / Samarium ion is in the fully trivalent state. ""* / / /

DC -so- T y / /

ID ~Y / /

2. Results. — The experimental techniques for ^ / / f

thermo-power and resistivity measurement at high 2 ~

^ " / / — ® 1 0 / ' 20 "

pressures have been described elsewhere [4]. Figure 1 ' / / °~-. / P (kbar) gives the different isotherms of thermopower (Q) 2 -120- / / "^-lOO- /

versus pressure (P) for SmS. It is clear that the magni- ^ / / — / tude of the thermo-power anomaly at the first-order X //

phase transition decreases with increase of tempera- ture. The inset in figure 1 corresponding to 835 °C

shows that the first-order phase transformation in "

° "

SmS has become continuous around this temperature.

We have also used the progressive narrowing of the

, . • • •.. • • . . „ „ „ Fig. 1. —Isotherms of thermo-power versus pressure for SmS.

pressure hystensis with increasing temperature as a

criterion to track the critical point. Figure 2 shows the

phase diagram of SmS constructed out of the present

thermo-power data. The solid line corresponds to the ing at a critical point which is around 825 °C. Further phase transformation pressures for the forward transi- the slope of the T-P plot is positive and has a value of tion and the dotted line to the reverse transformation. 170 °C/kbar.

The hysterisis interval closes around 825 °C which is The resistivity behaviour with pressure of consistent with the continuous variation of Q with Sm

Tb

S is shown in figure 3. It is seen that over a P at 835 °C (inset in figure 1). The present data gives narrow pressure interval of 5 kilobar, resistivity clear evidence for the S-M phase boundary terminat- decreases rapidly with pressure although at higher

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19795119

RESISTIVITY AND THERMO-ELECTRIC BEHAVIOUR O F SOME MIXED-VALENCE SYSTEMS C5-345

0 L 8 12 16 20

PRESSURE (kbar)

Fig. 3. - Relative resistance versus pressure for Smo,,Tbo,,S.

0 2 4 6 8 10 12

Sm0.8Tb0.2S

PRESSURE (k bar) -

1.0 Fig. 2. - Phase diagram of SmS from the present data.

-

temperatures this decrease becomes less rapid. Our previous data [5] shows that thermo-power anoma-

lously increases in the same pressure interval. The -

temperature behaviour of resistivity is shown in

figure 4. At low pressures the behaviour is similar

to that of a semi-metallic substance. However at

higher pressures, resistance increases linearly with TEMPERATURE ('C)

temperature, typical of a trivalent rare-earth chalco-

genide- These results indicate that at higher pres-

sures, Samarium ion is in the fully trivalent state.

References

[l] VARMA, C. M., Rev. Mod. Phys. 48 (1976) 219.

[2] JAYARAMAN, A., DERNIER, P. and LONGINOTTI, L. D., Phys.

Rev. B 11 (1975) 2783.

[3] TONKOV, E. Yu. and APTEKAR, I. L., F&. Tverd. Tela. 16 (1974) 1507.

[4] SHUBHA, V. and RAMESH, T. G., J. Phys. E 9 (1976) 435.

[5] RAMESH, T. G. and SHUBHA, V., Solid State Commun. 19 (1976) 591.